It is necessary to know the state of the core of a nuclear reactor, in order to guarantee safety (protection of the fuel assemblies) and proper operation of this reactor. For this purpose, it is possible to track several parameters related to the bulk power of the core: the linear power along the axis of the core, the CHFR (Critical Heat Flux Ratio), the axial and radial power disequilibrium, etc. Neutron detectors are used for reconstructing these parameters, because they depend on the neutron flux in the core.
Several types of neutron detectors are used:
detectors known as “excore chambers”, which are placed outside the core and which give a signal proportional to the mean flux in a quarter of the core;
“incore” detectors which are located inside the core. Certain incore detectors are mobile. They are periodically inserted into the core in order to establish a specific image of the power distribution in the core.
Other incore detectors are fixed and continuously give a signal representative of the local neutron flux in an area of the core of the reactor. The fixed detectors are permanently subject to irradiation, which in the long run causes a loss of sensitivity of these detectors and a degradation of the accuracy of the corresponding signal.
The use of cobalt neutron detectors as fixed incore detectors is known. These detectors behave like passive current generators, the current being generated by nuclear reactions within the detector under the effect of the neutron flux. Activation of cobalt 59 into cobalt 60 in the detector has the long-run effect of deteriorating the useful signal/total signal ratio of this detector, which is detrimental to the accuracy of the measurement.